In August, 10 people were injured after an American Airlines flight from Greece experienced "severe turbulence" prior to landing at Philadelphia International Airport.

In May, severe turbulence injured 27 passengers on a flight from Russia to Bangkok.

And in June, passengers traveling from Houston were carried off on stretchers.

So, what is causing this?

KARE 11 Meteorologist Sven Sundgaard says some of it may be attributed to "clear-air turbulence."

"This is where air is moving aloft," he says. "You can't see it, because there isn't a cloud developing. ... It's tricky for pilots sometimes, because you can't measure that or see that. You don't know until you run into it."

Sign Up for the Sunrise Morning Newsletter

Thanks for signing up!

Something went wrong.

This email will be delivered to your inbox once a day in the morning.

Please check your email to confirm your subscription.

Please try again later.

Sundgaard says clear-air turbulence is hard for meteorologists to predict.

"Without a visible sign, like a cloud, you just have to rely on reports coming from pilots," Sundgaard said. "Computer models can give us a general idea that there might be turbulence or an upward motion in the atmosphere over an area, but you never know where that turbulence can be and when."

A Southwest spokesperson would not say what the attendants on Flight 1233 were doing when they hit turbulence.

"Turbulence is scary as a passenger, but generally when you're aloft it's a pretty safe thing, as long as you're buckled in," Sundgaard says.

It can get dangerous for flight attendants, who are often walking around serving the seated passengers.

"When you have surprise turbulence like this, it can be a real danger for them," he says.

Some experts say climate change is changing air currents that planes encounter on routes between the U.S. and Europe, making for bumpier rides.